Method of liquid quenching glass sheets

ABSTRACT

An improved method of quenching heated flat glass is described which involves applying to a surface on or adjacent at least one peripheral edge of the glass prior to heating it, an edge protecting composition taken from the class of compositions having a low coefficient of expansion compared to glass and/or a heat transfer coefficient not greater than glass while maintaining the major surfaces inwardly of each of said surfaces on or adjacent said peripheral edge substantially free of said protective composition. The glass with its edge so treated is then heated to above its strain point and below its softening point (at which temperature range said composition fuses to the glass) and subsequently quenched using a liquid medium having a high heat transfer coefficient.

United States Patent [191 Seymour METHOD OF LIQUID QUENCI-IING GLASSSHEETS [75] Inventor: Samuel L. Seymour, Oakmont, Pa.

[73] Assignee: PPG Industries, Inc., Pittsburgh, Pa.

[22] Filed: May 15, 1972 [211 Appl. No.: 253,354

Related US. Application Data [63] Continuation-impart of Ser. No.171,324, Aug. 12

1971, abandoned.

[52] US. Cl 65/114, 65/104, 65/115,

' 65/116 [51] Int. Cl C03b 27/00 [58] Field of Search 65/104, 116, 114,

[5 6] References Cited UNITED STATES PATENTS 2,194,611 3/1940 Paddock etal. 65/114 X 2,198,739 4/1940 Phillips 65/116 FOREIGN PATENTS ORAPPLICATIONS 1,446,244 9/1965 France 65/116 [111 3,765,859 Oct. 16, 1973Primary Examiner-Arthur D. Kellogg Att0meyRussell A. Eberly [57 ABSTRACTAn improved method of quenching heated flat glass is described whichinvolves applying to a surface on or adjacent at least one peripheraledge of the glass prior to heating it, an edge protecting compositiontaken from the class of compositions having a low coefficient ofexpansion compared to glass and/or a heat transfer coefficient notgreater than glass while maintaining the major surfaces inwardly of eachof said surfaces on or adjacent said peripheral edge substantially freeof said protective composition. The glass with its edge so treated isthen heated to above its strain point and below its softening point (atwhich temperature range said composition fuses to the glass) andsubsequently quenched using a liquid medium having a high heat transfercoefficient.

15 Claims, No Drawings METHOD OF LIQUID QUENCIIING GLASS SHEETS RELATIONTO OTHER APPLICATIONS This application is a continuation-in-part ofcopending patent application Ser. No. 171,324, of Samuel L. Seymour,filed Aug. 12, 1971, for IMPROVED METHOD OF LIQUID QUENCI-IING, nowabandoned.

BACKGROUND OF THE INVENTION The tempering of glass is typicallyaccomplished by heating glass to an elevated temperature (a temperatureabove the glass stain point) and then suddenly chilling the glass tocool the glass surface rapidly while the interior regions of the glasscool at a slower rate. This differential cooling pattern of the glassbeing tempered results in the development of a compressive stress in theglass surface and a tensile stress in the interior regions of the glass.The resultant tempered glass has a much greater resistance to fracturethan does umtempered glass. Further, glass which has been tempered anddoes fracture has a breakage pattern which is significantly differentfrom that of untempered glass in that it shatters into small fragmentswith blunt rounded edges rather than large sharp-edged pieces whichresult when umtempered glass fractures. Tempered glass because of itsdesirable qualities on breakage has an obvious advantage over untemperedglass, rendering it particularly useful in transparent doors, motorvehicle closures, ophthalmic lenses and the like.

Glass has been tempered by applying cool air to the surface of heatedglass and has been found satisfactory for this purpose especially onglass of considerable thickness at inch or more). Cool air tempering hasnot been found as acceptable in cooling thinner glass (below /5 inch inthickness) in that only low degrees of temper have often been obtained.

It has been suggested in the prior art to replace air or gas as acooling medium and to use instead liquids. Liquids, becuase of theirsuperior heat transfer characteristics compared to air, should removeheat much more rapidly from glass surfaces than air. More rapid coolingwill set up a greater differential cooling pattern between the interiorand the surfaces of the glass cooled, thus creating the potential forthe attaining of a higher degree of temper in the glass than possiblewith air tempering.

Using liquids instead of air to thermally temper glass sheet isgenerally referred to in the art as liquid quenching. In liquidquenching, rapid cooling of the glass is accomplished by contacting thesurface of the glass with a liquid quenchant. Contacting the surface ofthe glass can be accomplished by various techniques, such as flowing aquenchingliquid over the surface of the glass. This is known as floodquenching and is described in U. S. Pat. application Ser. No. 108,661,filed Jan. 21, 1971. Another method of contact is spray quenching,"which involves atomizing the quenching liquid into discrete liquiddroplets and then spraying the surface of the glass. Spray quenching iswell known in the metal tempering art and is disclosed, for example, inU. S. Pat. No. 3,208,742. Another method of contact is to immerse theglass sheet completely in the quenching liquid. This technique isreferred to in the art as immersion quenching or dip quenching and isdescribed in U. S. Pat. Nos. 170,339, 2,145,119, 2,198,739, 3,186,816,3,271,207 and Belgian Pat. No. 729,055.

Although liquid quenching has been found to be moderately effective withrather thick glass sheets, it has not been very effective in temperingthinner glass specimens. Often the thinner glass fractures when thesheet is initially contacted with the quenching liquid. The fractureappears to initiate at the edge of the sheet, continues into the body ofthe glass resulting in total breakage of the sheet.

One of the more important causes of fracturing of thin glass sheets inliquid quenching processes is believed to be due to the fact that theedges of the glass cool at too rapid a rate when compared with theremainder of the glass sheet. In accomplishing a liquid quench bydipping a glass sheet into a liquid quenching bath, this rapid coolingof glass edges is magnified at the leading edge of the glass, i.e., theedge of the glass which first contacts the liquid quenching fluid. Thisdifferential cooling is referred to as the leading edge effect. Thus, indip quenching, this differential rate of cooling between the leadingedge and the remainder of the glass sheet puts the leading edgetemporarily in a tension stress greater than the tensile strength of theglass, which may cause fracture due to the thermal shock of dipquenching.

The leading edge cools at a greater rate than the remainder of the glasssheet because the leading edge dissipates heat by more mechanisms thandoes the remainder of the glass sheet. The leading edge dissipates heatnot only because of the temperature differentialbetween it and thequenching liquid, but also dissipates heat due to the hydrodynamicphenomena caused by liquid flow over the leading edge. This hydrodynamicflow of the quenching liquid over the leading edge as the glass sheetpasses through the quenching liquid on immersion removes tremendousquantities ofheat at a rapid rate from the leading edge. Thus, asituation is created where a relatively small area of the glass, that isthe leading edge, is cooled at a much faster rate than is the remainderof the glass. This difference in the rate of cooling puts the leadingedge in tension, in which condition it is particularly susceptible tothermal shock and breakage.

In accordance with this invention a technique is provided for thermallytempering glass by liquid quenching, which minimizes the problems ofpremature edge cooling and resultant glass fracturing.

SUMMARY OF THE INVENTION In accordance with the instant invention glasswhich is to be tempered is treated on at least one of the edge surfacesby applying thereto, prior to heating the glass above its strain point,an edge protective composition having a low coefficient of expansioncompared to the coefficient of expansion of glass and/or a heat transfercoefficient not greater than glass. The terms edge surface and edgesurfaces as used in the following description refer to the peripheraledge of a glass sheet and narrow areas of either major surface adjacenta periph eral edge of the glass sheet. The edge-treated glass is thenheated to an elevated temperature (above the strain point of theparticular glass composition and near but below its softening point). Atthis elevated temperature range, the edge protective composition meltsand fuses to the edge surface to form a protective edge coating. The hotglass is then cooled rapidly by contacting it with a liquid quenchingmedium having a high heat transfer coefficient to impart a high degreeof temper to the glass.

DETAILED INVENTION The compositions utilized in preparing one or moreedge surfaces of the glass to be tempered by liquid coolingin'accordance with one embodiment of this invention, have a thermalcoefficient of expansion less than the glass on which they are placed.Thus they typically have a thermal coefficient of expansion on the orderof 7.6 X 10 inches per linear inch per degree Centigrade (417.6 X l/C.), preferably 6.8 X inches per linear inch per degree Centigrade orless (046.8 X l0 /C.) measured at 0 to 300 Centigrade.

These compositions can be clear in appearance or can be colored.Typically the compositions contain silica, alumina, lithium, lead andboron. In the colored compositions the frits typically contain cobalt,chromium or mixtures thereof in addition to the above. In general thecompositions contain lead in concentrations of 45 to 52 percent byweight as lead oxide, silica as silicon dioxide at weight percentsbetween 33 to 37 percent by weight, alumina as aluminum oxide at weightpercent of from 3 to 4 percent, lithium as lithium oxide at 3 to 4percent by weight, boron as boron oxide at 4 to 5 percent by weight,chromic oxide ina range of 0.5 to 1 percent by weight and cobalt ascobaltous oxide (C0 0,) in a range of 1 to 2 percent by weight. Thepreferred frit compositions contain little or no alkali metal other thanlithium (less than 0.5 percent by weight) but typically contain lithiumoxide in quantities of at least 3 percent by weight of the fritcomposition. Similarly the preferred frit compositions contain leadmeasured as lead oxide in a quantity of at least 45 percent by weight.In some instances frits containing little or no lithium can be employed.In general frits of this character having a low thermal coefficient ofexpansion, contain large amounts of aluminum oxide 38 to 42 percent,typically 40 to 4i percent, and lead oxide in the'range of 33 to 36percent. The silica content of such frits is usually 16 to 20 percent.The remainder of such a frit is composed of material such as chromiumoxide and- /or cobaltous oxide in the range of 3 to 5 percent.

According to another embodiment of the present invention, a compositionhaving a heat transfer coefficient not greater than that of glass isapplied to the portion of the edge surface to be protected. Typical heatinsulating compositions of this other embodiment are pastes or cementswhose heat transfer coefficients do not exceed that of glass. Sinceglass has a heat transfer coefficient of 0.002 calories per-second persquare centimeter of area per centimeter of thickness per degreeCentigrade, any material that does not react chemically with glass ordecompose when subjected to the thermal treatment involved in temperingglass and whose heat transfer coefficient does not exceed that of glass,meets the requirements of this latter embodiment. Examples of suitablematerials to be included as components of compositions fulfilling therequirements of the latter embodiment include dispersions or pastes ofone or more pulverulent material such as lime, magnesia, magnesiumcarbonate, mica, calcium sulfate, silica, chalk, diatomaceous earth,lamp black and cerium oxide. Certain high silica content cements, suchas those available commercially as Sauereisen No. l cement paste andSauereisen No. 8 cement, for example, are especially suitable.

Edge protecting compositions having the characteristics of bothembodiments of this invention are especially preferred.

The frit is applied to the glass in any convenient manner. Typically thefrit is carried in an organic medium, rending the composition capable ofbeing brushed onto the glass surfaces to be coated. Any organic mediumwhich will thermally decompose at. the temperature employed in heatingthe glass above its strain point and in which the frit can be dispersedmay be used. Thus, material such as pine oil, crude oil, alcoholsethers, benzene, terpentine, chlorinated solvents such as chloroform,methyl chloroform, ethylene dichloride, carbon tetrachloride, and othersimilar materials are suitable. The frit is typically sized to providethat the particles are below 200 mesh in size and preferably percent ormore of the glass frit is below 325 mesh (U. S. Sieve Series). Ifdesired, viscosity modifiers can be used to render the carrier moreviscous. Utilization of brushes, rollers and other mechanical methods ofapplying the frit in the carrier vehicle are contemplated in placing thefrit on the glass edge surfaces.

After application of the frit composition to the edge surfaces of theglass, the glass is first heated to an elevated temperature, usuallysomewhere near the softening point of the particular glass beingtempered. The softening point, as used herein, is that condition inwhich glass has a viscosity of 10 poises. The temperature at thesoftening point of the glass will vary depending on the particualrcomposition of the glass. For example, in a soda-lime-silica glasscomposition, the temperature at the softening point is about 1400F. In aborosilica glass composition this temperature is about 1500F.

When the glass has been heated to the abovedescribed elevatedtemperature, it is immediately contacted with a liqud quenching agentwhere heat is exchanged between the surface of the glass and the liquid.A direct measure of this heat exchange is the heat transfer coefficient.The heat transfer coefficient is defined as the heat flux at theglass-liquid interface between the heated, immersed glass body and thesurrounding liquid per unit of temperature difference-unit of time-unitof glass surface. For the purposes of this invention, the heat transfercoefiicient is expressed as British Thermal Units/hour-squarefoot-degree Fahrenheit (B.T.U./hr.-ft.-F.).

In tempering the treated glass articles, the quenching liquid preferablyis one which provides a relatively high average heat transfercoefficient over the entire range of temperature employed in thetempering process. With thin glass, heat exchange between the interiorof the glass and the surface is appreciably more rapid than with thickerglass. Therefore, in order to achieve as high a degree of temper in thinglass as is obtainable with thicker glass, the heat flux at theglass-liquid interface must be proportionately greater for thin glassthan for thicker glass. One way of providing a greater heat flux at theglass-liquid interface is to quench in a liquid providing a relativelyhigh average heat transfer coefficient over the entire temperature rangeemployed in the tempering process.

It has been found in the practice of this invention in tempering glasspreviously treated on one or more edge surfaces with frit having a lowthermal coefficient of expansion by a liquid-quenching technique, thatthe quenching should be accomplished in liquids which have an averageheat transfer coefficient of at least 125 B.T.U./hr.-ft.-F., preferablywithin the range of 200 to 500 B.T.U./hr.-ft. -F. as the glass is cooledthrough the temperature range used in the tempering process. Thetemperature range used inthe tempering process is defined as the surfacetemperature of the glass near its softening point down to a lowersurface temperature at which the interior of the glass has cooledthrough the glass strain point. The glass strain point as used herein isthat condition in which glass has a viscosity of poises. When glass hasbeen cooled completely through the strain point, the final degree oftemper in the glass has been attained.

In tempering thinner glass, i.e., glass of about 0.050 to 0.090 inch inthickness, to obtain a high degree of temper, quenching liquids shouldbe used which provide an average heat transfer coefficient in the upperranges of the limits set forth. With thicker glass, i.e., glass of about0.100 to 0.500 inch in thickness, a high degree of temper can beobtained using liquids which provide average heat transfer coefficientsin the lower ranges of the limit set forth.

Besides quenching with liquids which provide an average heat transfercoefficient at the glass-liquid interface within the above-specifiedlimits, it is also important that the quenching liquid provide arelatively high actual heat transfer coefficient as the glass is cooledthrough the lower temperature regions of the tempering process. Theselower temperature regions are defined as the range of glass surfacetemperatures from near its strain point down to a lower surfacetemperature when the interior regions of the glass pass through thestrain point.

In the practice of this invention, it has been found in tempering theedge treated glass that the actual heat transfer coefficient at theglass-liquid interface should not fall off too quickly as the glass iscooled down through the lower temperature regions of the process.Accordingly, the heat transfer coefficient should be always at least125, generally 125 to 600, and preferably 200 to 500 B.T.U./hr.-ft. -F.as the glass surface is cooled from a temperature near its strain pointdown to a lower surface temperature at which the center of the glass hascooled to a temperature below the strain point. As an example, withsoda-lime-silicate glass, the temperature of the glass surface at thestain point is typically about 960F. The lower temperature, i.e., thesurface temperature of the glass at which the center of the glass hascooled to a temperature below the strain point is more difficult to pindown and will depend upon, among other things, the thickness of theglass, the composition of the glass, and the heat flux at theglass-liquid interface. For soda-lime-silica glass having a thickness ofabout 0.050 to 0.125 inch with a heat transfer coefficient at theglass-liquid interface which will provide for a high degree of temper inthe resultant glass article, the lower temperature of the surface of theglass will be about 500600F.

Examples of liquid-quenching agents useful in the practice of thisinvention are the polyoxyalkylene glycols, such as polyoxyethyleneglycols, polyoxypropylene glycols or mixtures thereof (polyglycols).

Polyoxyethylene glycols, HO- CH,CH,(OCH,CH,),OH, n l to 1000, range atroom temperature from water-white liquids to waxy solids. Those above1000 in molecular weight are sold commercially under trademark CARBOWAX.Although pure polyoxyethylene glycols can be prepared and can be used inthe practice of this invention, the commercially available compounds areactually mixtures of a number of polyoxyethylene glycol polymers ofvarious molecular weights. The commercially available polyoxyethyleneglycols up to a molecular weight of about 700 are water-white liquids atroom temperature. Those having molecular weights of 1000 or above aresolids that vary in consistency at room temperature from apetrolatum-like grease to a hard wax. If these higher molecular weightpolyoxyethylene glycols are used as the quenching liquids in thepractice of this invention, they should first be heated to above theirmelting point to convert them to a liquid before use. Thepolyoxyethylene glycols are typically prepared by an alkali catalyzedcondensation of ethylene oxide in water:

Polyoxypropylene glycols HO(C HO),,C li -OH, n l to 1000, are colorlessto light yellow viscous liquids. They are commercially available to mo-;lecular weights ranging from approximately 400 to 2,000. They aremanufactured by the polymerization of propylene oxide in the presence ofan acid or alkaline catalyst. The procedure is similar to that used inthe preparation of polyoxyethylene glycols. The low molecular weightpolyoxypropylene glycols, i.e., up to a molecular weight of 500, arecompletely water soluble, while those of higher molecular weight, i.e.,from 1,000 to 2,000, are only slightly soluble in water. Those having anintermediate molecular weight, i.e., from 600 to '900, are moderatelysoluble in water, about 10 to 20 percent at room temperature. For thepractice of this invention, the partially and completely water solublepolyoxypropylene glycols are preferred. Polyoxypropylene glycols arecommercially available under the trademark NIAX.

Mixed polyoxyethylene-polyoxypropylene glycols are hydroxy-terminatedcopolymers of ehtylene oxide and propylene oxide. These mixed glycolsrange from mobile to very viscous liquids with molecular weights as highas 40,000. Either water-soluble or waterinsoluble products can beprepared, although watersoluble products are preferred in the practiceof this invention. Mixed polyoxyethylene-polyoxypropylene glycols aresold commercially under the trademark UCON. 7

Other liquids which can be utilized in the instant process are siliconefluids and hydrocarbon oils.

The silicone fluids of the invention are dialkyl, diaryl or alkyl-arylfluids. The fluids are linear polymers of alternating silicon and oxygenatoms, each silicon atom having two organic groups attached to it. Thechemical formula for the silicone fluids is:

L in

where n varies from 9 to 100, and R is selected from alkyl and/or arylgroups. R can be selected from lower alkyl groups having l to 4 carbonatoms. When R is methyl, the above formula depicts the familiar dimethylsilicone fluids. Also R can be selected from aryl groups, especiallyphenyl groups and alkyl and halo substituted phenyl groups. Theincorporation of phenyl groups into the polysiloxane polymer backboneincreases the oxidative stability of the fluid. The larger the value ofn in the above formula, the higher the molecular weight and the higherthe viscosity is in the resulting silicone fluid. in the practice ofthis invention, silicone fluids having a molecular weight of from about675 to 11,000 and a viscosity of from about 5 to 200 centistokes at 77F.are contemplated.

Silicone fluids are sold commercially under the tradenames Dow CorningSilicone Fluids and General Electric SF. Silicone fluids are well knownin the art and are described in An Introduction to the Chemistry of theSilicones,2nd Edition, by E. G. Rockow, Wiley, New York 195 l One methodof preparing the silicone fluids useful in the invention is to hydrolyzein an acidic-aqueous meers, thermal stabilizers, viscosity modifiers,surfactants and the like. Such mineral oils are well known in the art ofmetal tempering and are further described in detail in MetalworkingLubricants; Their Selection, Application and Maintenance, by E. L.Bastian, McGraw-l-lill, New York 1951 and Lubricants and Cutting Oilsfor Machine Tools by W. G. Forbes, Wiley, New York 1943.

The liquid quenching mediums of this invention should be essentiallywater free. By essentially water free is meant that the quenching mediumcontain less than 5 percent by weight water. if greater amounts of waterare present, there will be an increased tendency for the glass sheet tofracture during the tempering operations. This problem is particularlyserious with thinner glass sheets, i.e., sheets having a thickness ofabout 0.050 to 0.090 inch. Although not intending to be bound by anyspecific theory, it is believed that water forms a vapor blanket aroundthe glass sheet, when the glass sheet is initially contacted withquenching medium. This vapor blanket acts as an insulating barrier andinhibits heat flux at the glass-liquid interface, heat being removedprincipally by radiation through the dium dialkyl, diaryl and/or alkyl,-aryl dichlorosilanes,

Reduction of the amount of trialkyl silane allows, of course, theformation of higher molecular weight compounds. This end blockingtechnique is an efiective way to control viscosity and also serves tostabilize the viscosity of the silicone fluid against furtherpolymerization upon standing.

The hydrocarbon oils useful in the practice of this invention are theso-called heat treating oils which are will known in the art of metaltempering. Such oils are mineral oils which are obtained from cokingoperations or petroleum refining. The oils are characterized in thatthey have low volatility, have resistance to hightemperature, haveresistance to oxidation, and have a high flash point. More specifically,the mineral oils useful in the practice of this invention should have aboil-.

ing range from about 400 to 800F.; a flash point from above about 300 to500F. and a SUS viscosity within the range of from about 100 to 2500seconds at 100F. if the mineral oils used are petroleum .derivatives,they may be parafi'mic base aromatic base, i.e., naphthenic or mixedbase oils. Present in the mineral oil, of course, can be variousadditives such as anti-oxidants, emulsifivapor film. Thus, the glass isinitially cooled very slowly and verylittle potential for temper isdeveloped. Eventually enough heat will be removed by radiation such thatthe glass has cooled to a temperature near its strain point. This is atemperature at which the glass ceases to behave as a viscous liquid andstarts behaving as an elastic solid. At about this temperature, thevapor blanket is no longer stable and the water comes in direct contactwith the glass surface and violent boiling ensues. Heat is removed fromthe surface of the glass as latent heat of vaporization at a very rapidrate. However, since the glass surface is starting to behave as a solid,it cannot withstand this rapid heat flux and as a result the glassshatters.

The liquid quenching media ofv the instant invention can contain variousadditives, such as viscosity modifiers, suspension and emulsionstabilizers, wetting agents, detergents, anti-oxidants and thermalstabilizers. Examples of such additives include carboxy methylcellulose, sodium alkyl sulfonate, sodium dioctyl sulfosuccinate andtert-butyl catechol.

7 ln utilizing the liquid quenching baths of the instant inventiontemperature of the bath can vary considerably. Thus, bath temperaturesmay be ambient (70F.)

.or they may be above or below ambient temperatures.

Typically the temperatures range between F. to 4S0C.- lt has been foundwith larger glass pieces that heating the quenching liquids in the rangeof 200F. to

450F. produces considerably less breakage than temperatures below thisrange. The upper practical limit on the temperature used is usually theflash point of the particular quenching liquid or blend of quenchingliquids used.

The various liquid quenching fluids may be utilized alone or may beblended with one or more liquids to provide the liquid quenching fluidto be employed.

The glass which is tempered according to the method of this invention istypically a silica-type of flat glass, particularly soda-lime-silicaglass, lead-silica glass and borosilica glass. The nature and productionof silica glasses is well known in the art, and generally is describedin the Encyclopedia of Chemical Technology, by Kirk-Othmer, published byInterscience Encyclopedia In., New York, N. Y. volume 7, pages 181-189.The

method of the invention has been found to be particularly useful in thetempering of thinner glass sheets, i.e., glass sheets having a thicknessof about 0.050 to 0.125 inch. The method of the invention is generallyapplicable to tempering glass sheets of from 0.050 to 1 inch or more inthickness.

The geometric configuration of the glass treated in accordance with thisinvention is not particularly critical in that flat-glass sheets andcurved-glass sheets, for example, curved-glass Windshields, may betempered using the quenching fluids hereinabove described. The instantprocess may be readily adapted to either a continuous or asemi-continuous operation to produce resultant tempered glass sheetshaving an abraded resistance to fracture by stress of the order of 5,000to 40,000 pounds per square inch and a central tensile stress of theorder of 2,500 to 20,000 pounds per square inch.

The edge surfaces of the glass to which the frit is applied may betreated by grinding to seam them if desired. The edge surfaces when sotreated are typically ground with a diamond grit wheel and finish groundusing a belt grinder. if desired however, the frit can be applied to thenew cut, edge surfaces of glass-sheets without any seaming by grindingand glass successfully tempered using a liquid quenching medium, whensmall sheets are treated. Y

The following examples are illustrative of the practice of the instantinvention in tempering glass sheet. Examples 1-4 use glass edgeprotecting compositions having a lower thermal expansion coefficientthan glass, while compositions with lower coefficients of heattransmission provide edge protection in the other examples.

EXAMPLE 1 A soda-lime glass sheet 16 inches by 42 inches by 0.090 inchwas edge seamed using a diamond grit grinding wheel. The edges were thenwet belted with a 400 grit belt.

Glass frit composition particles all below 325 mesh in size having thechemical composition listed in Table l and carried in a pine oil vehiclewere applied to the edge surfaces of the glass sheet after the wetbelting. The glass after having all edge surfaces coated with the fritcomposition was heated to a temperature of 1230F. Upon attaining thistemperature the glass was shock chilled by dipping it in a quenchingliquid composed of a polyoxyalkylene oxide designated as UCON HB 5100manufactured'by Union Carbide Corporation. The quenching liquid beforeimmersion was at a temperature of 350F. The treated glass sheet did notbreak on immersion and subsequent observation of the cooled glass showedit to be tempered.

TABLE 1 Frit Composition wl.% SiO; 18.52 Al,0, 40.08 (150, 4.52 PbO35.04

EXAMPLE 2 Using the frit composition of Table 11 with all particlesbelow 325 mesh and carried in a pine oil carrier the edge surfaces of asoda-lime glass sheet 12 inches by 12 inches by ,4. inch which had notbeen seamed as TABLE I1 Frit Composition a wi rem ves SiO, AI,O, L =08,0, C O, C0 0 amoqaaweme I EXAMPLE 3 A soda-lime glass sheet 12 inchesby 12 inches by /8 inch was edgeseamed using a diamond grit grindingwheel. The edges were then wet belted with a 400 grit belt.

Using the frit composition of Table III with the frit particles beingbelow 325 mesh and carried in a pine oil vehicle, the edge surfaces ofthe soda-lime glass sheet after wet belting were coated completely withthe frit. The glass was then heated to a temperature of 12309F. Uponreaching this temperature the glass was immersed in a UCON HB 5100quenching bath which was at a temperature of about 120F. UCON HB 5100 isa polyoxyalkylene oxide manufactured by Union Carbide Corporation. Thecooled glass sheet after removal is found to be satisfactorily temperedand shows no signs of thermal shatter.

TABLE 111 Frit Composition Wt.% SiO, 36.87 A1 0, 3.94 Li O 3.64 PbO50.71 8,0; 4.85

prebent 0.090 inch thick windshield glass and the glass has beensuccessfully tempered in a polyoxylakylene glycol liquid quench withoutshattering.

EXAMPLE 4 The processes recited in Examples 1 to 3 were followed usingthe frit composition of Table l as an edge protective compositionapplied to the leading edge portion comprising its leading edge surfaceand 4 to 5 inches along the side edge surface extending away from theleading edge of a pattern for a Volkswagen windshield approximately 16inches by 42 inches by 0.090 inch and a pattern for a windshield for aFisher Body approximately 24- inches by 58 inches by 0.090 inch. In eachinstance, the glass after being coated along the leading edge portion asrecited hereinbefore was heated to a temperature of approximately l230F.Immediately upon attaining this temperature, the glass was first shapedby press bending to a desired windshield pattern nd shock-chilled bydipping it in the quenching liquid recited in Example 1 at a temperatureof 350F. to 450F. The treated glass sheets did not break on immersionand inspection of the sheets after cooling showed that they developed atemper.

' EXAMPLE Additional samples of the windshield patterns of the typesthat were treated by a ceramic frit in Example 4 were treated along theleading edge surface and 4 to 5 inches of the leading portion of theside edge surfaces in a direction away from the leading edge surfacewith a composition obtainable commercially as Sauereisen No. l CementPaste. This composition has a coefficient of heat conductivity that isless than that of glsss. An analysis of the Sauereisen No. l CementPaste is contained in Table 4.

After applying the protective composition (Sauereisen No. l CementPaste) to'theleading edge portion of the glass sheet, the glass washeated to a temperature of approximately l230 F. and then immediatelyquenched in a bath of UCON 75 H 90;000, a more viscous polyoxyalkyleneoxide than UCON H8 5 W0, maintained at a temperature of between 380F.and 385F. The cooled glass sheets after removal were found to besatisfactorily tempered and showed no sign Qualitative EmissionSpectrograph Analysis Major lngredent Si Minor Ingredients Zn, Al, Fe

Trace to Minor Zr. Li, Sn

Trace Ca, Mg, Pb, Cr, Mn,

Ni. Ag. Ou

(Note: The presence of Zn interferes with the dtection and estimation ofNa) EXAMPLE 6 Glass sheets having the same dimensions as those treatedin Example 5 were treated in the same manner as the samples treated inExample 5 except that the leading edge portions including the 4 to 5inches along the side edge surface disposed adjacent the leading edgewere initially treated with a coating of a paste of finely dividedcerium oxide and water before the samples were heated and quenched andothers were treated around the entire edge surface with the cerium oxidepaste. The treated sheets were immersed in UCON 75 H 90,000 at atemperature between 350'F. and 400F. after said heating. The cooledglass sheets after removal were found to be satisfactorily tempered andshowed no sign of thermal breakage.

Thus, while the invention has been described with reference to certainspecific embodiments, it is not intended that it be limited therebyexcept insofar as appears in the accompanying claims.

I claim:

1. A method of quenching heated flat glass to provide tempered glasscomprising applying to at least the peripheral surface of at least oneedge of the glass prior to heating to a temperature near its softeningpoint a composition capable of fusing or adhering to glass on exposureto a temperature above the strain point of the glass to form an edgeprotective coating while maintaining the major surfaces of said glasssheet inwardly of each of said surfaces on or adjacent said peripheraledge thereof substantially free of said coating, said coating beingapplied in sufficient amount to inhibit the establishment in said edgeof tension stress sufficient to make the glass susceptible to breakageduring said quenching, heating the glass with said composition appliedto each said surface on or adjacent each said peripheral edge andsubsequently quenching the glass in a liquid quenching medium.

2. A method of quenching heated flat glass to provide tempered flatglass according to the method of claim 1, comprising applying to atleast the peripheral surface of at least one edge of the glass prior toheating a frit having a low melting point and a coefficient of thermalexpansion of approximately 7.6 X 10" per degree centigrade or less,heating the glass and subsequently quenching the heated glass in aliquid quenching medium.

3. A method of tempering flat glass according to the method of claim 1,comprising applying to at least the peripheral surface of at least oneedge of the flat glass a low melting frit having a coefficient ofthermal expansion of approximately 6.8 X 10' per degree Centigrade orlower, heating the glass to near its softening point and quenching theglass so heated by contacting it with a liquid heat transfer fluid torapidly chill the glass to a temperature below its strain point.

4. The method of claim 1 wherein the quenching liquid is taken from thegroup consisting of oxyalkylene polymers, silicones, hydrocarbon oilsand mixtures thereof.

5. The method of claim 1, wherein the glass is heated to temperaturesabove its strain point and near its softening point and is chilledduring quenching to temperatures below the strain point. 6; The methodof claim 2 wherein the frit has on a weight-percent basis thecomposition:

SiO 33.8

Li O 3.34

PbO 46.50

7. The method of claim 2 wherein the frit has on a weight percent basisthe composition:

SiO, 18.52

Ago. 40.08

PbO 35.04

8. The method of claim 2 wherein the frit has on a weight percent basisthe composition:

SiO, 36.87

ALO, 3.94

PbO 50.71

9. The method of claim 3 wherein the frit has a coefficient of thermalexpansion of approximately 6.8 X -8.

10. The method of claim 3 wherein the liquid heat transfer fluid istaken from the group consisting of silicones, oxyalkylene oxides,oxyalkylene glycols, hydrocarbon oils and mixtures thereof.

11. The method of claim 1, wherein the liquid quenching medium is attemperature of between about 200F. to about 450F.

12. The method of claim 1 wherein the composition applied to the glassedge surface is taken from the group of compositions having a heattransfer coefficient not greater than that of glass.

13. The method of claim 1, wherein said composition is taken from thegroup of compositions consisting of dispersions or pastes of one or morepulverulent materials taken from the class consisting of lime, magnesia,magnesium carbonate, mica, calcium sulfate, silica, chalk, diatomaceousearth, lamp black and cerium oxide.

14. The method of claim 1, wherein the composition applied to the glassedge surface is taken from the group of compositions having a lowcoefficient of thermal expansion compared to glass.

15. The method of claim 4, wherein said composition is a glassy frithaving a melting point less than that of said flat glass.

2. A method of quenching heated flat glass to provide tempered flatglass according to the method of claim 1, comprising applying to atleast the peripheral surface of at least one edge of the glass prior toheating a frit having a low melting point and a coefficient of thermalexpansion of approximately 7.6 X 10 6 per degree centigrade or less,heating the glass And subsequently quenching the heated glass in aliquid quenching medium.
 3. A method of tempering flat glass accordingto the method of claim 1, comprising applying to at least the peripheralsurface of at least one edge of the flat glass a low melting frit havinga coefficient of thermal expansion of approximately 6.8 X 10 6 perdegree Centigrade or lower, heating the glass to near its softeningpoint and quenching the glass so heated by contacting it with a liquidheat transfer fluid to rapidly chill the glass to a temperature belowits strain point.
 4. The method of claim 1 wherein the quenching liquidis taken from the group consisting of oxyalkylene polymers, silicones,hydrocarbon oils and mixtures thereof.
 5. The method of claim 1, whereinthe glass is heated to temperatures above its strain point and near itssoftening point and is chilled during quenching to temperatures belowthe strain point.
 6. The method of claim 2 wherein the frit has on aweight percent basis the composition: SiO2 33.8 Al2O3 3.61 Li2O 3.34 PbO46.50 B2O3 4.45 Na2O 8.30
 7. The method of claim 2 wherein the frit hason a weight percent basis the composition: SiO2 18.52 Al2O3 40.08 Cr2O34.52 PbO 35.04
 8. The method of claim 2 wherein the frit has on a weightpercent basis the composition: SiO2 36.87 Al2O3 3.94 Li2O 3.64 PbO 50.71B2O3 4.85
 9. The method of claim 3 wherein the frit has a coefficient ofthermal expansion of approximately 6.8 X 10
 6. 10. The method of claim3, wherein the liquid heat transfer fluid is taken from the groupconsisting of silicones, oxyalkylene oxides, oxyalkylene glycols,hydrocarbon oils and mixtures thereof.
 11. The method of claim 1,wherein the liquid quenching medium is at temperature of between about200*F. to about 450*F.
 12. The method of claim 1, wherein thecomposition applied to the glass edge surface is taken from the group ofcompositions having a heat transfer coefficient not greater than that ofglass.
 13. The method of claim 1, wherein said composition is taken fromthe group of compositions consisting of dispersions or pastes of one ormore pulverulent materials taken from the class consisting of lime,magnesia, magnesium carbonate, mica, calcium sulfate, silica, chalk,diatomaceous earth, lamp black and cerium oxide.
 14. The method of claim1, wherein the composition applied to the glass edge surface is takenfrom the group of compositions having a low coefficient of thermalexpansion compared to glass.
 15. The method of claim 4, wherein saidcomposition is a glassy frit having a melting point less than that ofsaid flat glass.